OBJECTIVES: Development of a simplified cytogenetic assay system of chemical mutagens. From our previous protocol we shall attempt a shortened version of cytogenetic screening system which is aimed to obtain useful data on each chemical within two days. We shall continue the use of CHO cells as test object and test at least 100 compounds, known clastogens and unknown clastogens. There will be two treatment schedules, one for continuous treatment for 5 hrs and the other, for 4 hrs and returned to normal growth medium for 24 hrs. In each sample 50 metaphases will be read for the enumeration of chromosome lesions. If this procedure is too simplified, adjustments will be made. 2. Bleomycin effects on cellular DNA and chromosomes. This project will be completed in the fiscal year 1977-1978. We believe that the chromosome dissolution phenomenon is a unique one, and that it is related to the bleomycin effect. This project is more of fundamental interest than of practical value. 3. Effects of environmental mutagens on gametogenesis. This project will take at least two years. Chinese hamsters will be used as the principal material. We plan to test the effects of clastogens and mitotic poisons on all stages of meiosis. At the beginning, only known clastogens (ionizing radiation, MNNG, Actinomycin D) and mitotic poisons (halothane) will be used to perfect the protocol.